Method of electroplating zinc on iron or steel



March 3, 1931. s. H. DAVIS 'r AL 1,795,079

METHOD OF ELECTROPLATING ZINC ON IRON OR STEEL Filed Sept. l2, 1928 ZINCCHLC//VE MENGE/V555 wax/0E (fram cef/j To @xm/ZE EEE/vous fao/v wsoLaT/a/v SLI/ME LSTEEL fnv@ /Ho/v PHaauc-rl LAL frfu. 1 c1.EmvE/-GREHSE REMO ym. l

WASH WRTEP [nc/n P/c/rL/NG oF STEEL n/va UPON PRODUCTS] WHS H WHTEI? A,

CELL FR 5TH/KE C0197' OFZ/NG- m INSOLUBLE .NGDES-LW HCI T/ITE' CORTEZSTEEL /V /IYGN POD UCTS OPERA-UNG CELL -lrvso/.ueLEH/VODES-J -d'TE ELO77 /RoA/Ffoucrs 4s cnn/0065 -H/GH Hc/D SOLUTION I/ z/Nc conTE STEEL.nwo /no/v Pn'oaucrs l WITNESS Br FITTOH/YEYS Patented Mar. 3,I 1931UNITED STATES PATENT OFFICE I 4 SIDNEY H. DAVIS AND CARL O. ANDERSON, OFBAXTER SPRINGS, KANSAS, AND WIL- LIAM N. SMITH,' 0F PLATTEVILLE,WISCONSIN, AND HERBERT R. HANLEY, 0F ROLLA, MISSOURI, ASSIGNORS T THECENTURY ZINC COMPANY, A CORPORA- rIoN or onio METHOD oF ELECTEOPLATINGzINc oN IRON on STEEL Application filed September 12, 1928. Serial No.305,429.

The object of our invention, considered .iii its broadest aspect, is toprovide a cyclic method by which zinc from a commercial raw vsourcevmaybe directly plated on an iron or 'on the article is effected bysubmerging; the latter in a neutral or substantially neutralV solutionof zinc sulfate or an alkaline solu tion of cyanide of zincand thenpassing'an electric current of relatively low density from a solublezinc anode, likewise submerged in the bath, to the article actingv as acathode. Both of these processes necessarily require initial productionfrom the zinc ore of metal-1 lic Zinc, either for the preparation of themolten bath or to serve as the soluble anode in the plating bath. Whenthe zinc or spelter is to be used for hot dipping, the most satisfactoryresults are obtained when'the metallic zinc isQ recovered from the oreby the well known retort process, experiments having shown that thesubstantially pure metallic zinc derived from the ore by theelectrolytic processes now in use is unsatisfactory for hot dipping. Inlike manner the metallic zinc to be used as the soluble anode in theelectroplating process is ordinarily produced by the retort process butirresp ,ctive of whether it be produced by the retortprocess orelectrolytically, initial productionofthe metallic zinc a's such isrequired asa preliminary to its use in the electroplating process, sothat in either of the methods hitherto employed forl coating iron orsteel articles *withl zinc, .the latter must irst be produced by oneproc- 'ess andithen utilized in an'entirely separate and differentprocess, i. e., hot dipping or electroplating -for coating the article.In accordance with the preferred practice of our method however, we areable in a single cycle comprising a series ofvinter-rezinkiferousmaterial to the article to' be plated, thus entirely avoiding thenecessity for rst producing metallic zinc by one series of operationsand then utilizing that zinc to form the coating upon the article inanother and separate series of operations.

The preferred practice of our method is thus advantageousl in numerousrespects,

among Which'may be mentioned marked economy in operating costs; a higherpercentage of recovery of zinc from the concentrate than has been usualin the methodsof recovery heretofore practised, the loss of zinc by ourmethod being less than one-third of that occurring in the combination ofretort recovery and hot dipping now in use; permissible recover ofley-product metals such as cadmium and lead contained in the ore whichare now wasted jor lost when metallic zinc is recovered by the retortmethod but which have a present value of several dollars lper tonof ore.Moreover, we are able in accordance with our invention. to produce uponthe article a; coating of markedly different and superior:;characteristics to that which it has'beenl-:p' vQ'ble to produce eitherby the hot dipping"relectroplating process as heretofore practised.

. Other advantages and novel features and characteristics of ourinvention are hereinafter more specifically mentioned or will beapparent to those skilled in the art as a more detailed comprehensionthereof is acquired from the following description.

As stated, our method is preferably a cyclic one inwhich, among otherthings, zinc calcines derived from raw zinc concentrates and/or otherzinkiferousV materials are continuously leached with an .acid solutionsupplied from the cell or cells in which the plating ofthe article iseffected, and which4 for convenience may be termed the operating cells,so as to produce a zinc sulfate solution which, after suitabletreatment, is 'fed inv regulated quantity to the plating bath so as tomaintain the bath, and in which the article to be coated and acting asthe cathode and an insoluble anode are suspended, in a relatively highlyacid condition while a current of relatively highdensity is passed fromthe anode to the cathode to eiect the deposition thereon the bath, theprocess is a truly cyclic one, it`

being only necessary during its operation to continuously supply asufficient quantity of the zinc calcine or other source of zinc tocompensate for the amount of metallic zine deposited on the ferrousarticles.

In vthe use of an insoluble anode resides an important anddistinguishing difference between our method and the ordinaryelectroplating processes, enabling us as it does to achieve andsatisfactorily utilize plating baths of relatively high acidity andcurrents of high density. In the ordinary electroplating practice, onthe other hand, soluble anodes are employed Which prohibit the use ofrelatively high acid solutions or baths since the same by dissolvingVthe anodes would destroy or materially lower their own acidity.

By the use of insoluble anodes, however, this result is obviated, and weare thereby enabled to provide a process in which the acid generated inthe operating cell or cells is desirably utilized for dissolving thezinc ealcine or other zinc material serving as the source of the zinewhich is ultimately plated on the ferrous article by the utilization ofhigh density currents with resulting production, through the use ofsuchcurrents in connection with high acid solutions, of a coating or platingmarkedly superior to those which it has been possible to heretoforeproduce by any method or process with which We are familiar.

The deposition of zinc froinliiglily acid p zinc sulfate solutions oniron or steel articles acting as cathodes isrfrst attended by theevolution and deposition of hydrogen at the artlcle or cathode. After avariable interval of. time, the exact length of which differs inaccordance with voperating conditions and the like the major evolutionof this hydrogen ceases andl the cathode is found to be incipientlycoated with a thin deposit of zine but during the formation of thisinitial or incipient coating while the major evolution of the hydrogenis taking place the current efficiency is relatively lowl withIresultant diminution of operating economy., However, after theformation'of the initial coating and cessation of the major evolutionofv hydrogen, the current eiiciency increases and the zmc 1s deposltedmore readily.

We have found the major evolution of hydrogen can be checked and aninitial or strike coating of zinc formed on the article in a giveninterval of time after it has been rendered properly vzinc-receptive bypickling and brushing and/or other surface conditioning inthe usual orany suitable way, by the use of a current whose density .varies almostdirectly with the acidity of the solution or electrolyte; thus, forexample, if'the evolution of hydrogen and initial deposit of zinc can beeffected on an iron or steel article having a given type of surface in30 seconds when using a current density of 100 amperes per sq. ft. ofcathode surface and a solution containing 5% sul- -phuric acid, it isrequisite to effect the same result when employing a solution containing10% sulphuric acid, thatis, a solution having twice the acidity of thefirst solution, to employ a current having a density of substantially200 amper-es per sq. ft. of cathode surface. From this it follows thatf.if iron or steel articles are to be coated with zinc to a thicknesssufficient forordinary cominercial uses in a reasonable length of time,it is necessary when utilizing relatively high acid solutions to employ'currents of relatively Vgreat density necessarily requiring eX- tremelyheavy busbars, wiring and other instrumentaliti s for theirtransportation with resultant enhincement of the cost of initialinstallation and maintenance as Well as of fixed charges during the lifeof the plant.

Under certain conditions this may not be found objectionable but inaccordance with the preferred practice of our invention We consider itgenerally more advantageous to effect an initial or strike coating onthe iron or steel article by means of a current of very high density andat high efficiency and to then form a subsequent coating by using acurrent of relatively lower density but with equal e''iciencywith theresult that since the high density current is employed but momentarilyit is unnecessary to provide means for transportingand handling it ofthe same capacity and size as would be the case were the said currentemployed for the longer interval necessary to form a thicker' deposit;additionally, as this practiceA materially shortens the period duringwhich the evolution of hydrogen takes place in large quantities fromthesteel or iron article acting as a cathode, the production of an ultimatecoating of great perfection and high quality is I derived, reference inthis connection being l Madere.

had the accompanying drawing showing a flow sheet therecf, the method,however, `being capable of modification in many particulars withoutdepartingfron'i, the spirit and scope of the invention, for example as-will' hereinafter appear, by the employment of metallic zinc instead ofraw zinc coricen trates as the source of zinc supply.

Pursuant to the preferred practice offth'e present method, zinc calcinesderived by roasting or other suitable treatment of the raw zincconcentrates arev continuously leached with an acid solution derivedfrom the cells in which the articles are plated so as to produce a zincsulfate solution which, after suitable treatment, is fed in regulatedquantity to one or more of said cells so vas vto maintain the bathstherein, in which the said articles acting as thecathodes and insolubleanodes are suspended, in arelatively highly acid condition while `acurrent isbeing passed therethrough from the anodes to the articles toplate thereon metallic zinc supplied from the baths as distinguishedfrom a soluble zinc anode as in the ordinary electroplating process,atleast two'of such source from which the zinc is obtainedr totherebyform fresh zinc sulfate solution containin' ya chemicallyequivalentquanti-ty of zinc w ich in turn is fed back'to the cells, theprocess is a truly 'cyclic one, it being' onlynecessary during itsoperation to lcon-l tinuously supply a suiiicient quantity of the' zinccalcines or other zinkiferous material to make up foi' the metalliczincultimately abf stracted from vthe operation in the Yforni of coatings onthe iron and/or steel articles. Thus, when employing the raw zincconcentrate,1the samemay first beroastedin the :usual lWay so as tooxidize thezinc sulphide into zinc oxide and, to some extent, into .zincsulfate which compounds a'resolublein d1lute sulphuric acid and invvwater' respectively.

i* i The zinc calcines produced by the ALroasting operation are thenleached with a dilute solution of sulphuric acid which, as hereinafter 3,explained,.is.drawn :from the cells. Due to -the immersion of the ironand steel articles in the plating baths 'or from other causes, thissolution may contain some iron which it is necessary to remove,conveniently by first oxidizing the iron sulfate from the ferrous to theerric state with some eiective and permissible oxidizer, for example,manganose dioxide (hydrated) which is producedl as a by-product `fromthe anodes in the strike and operating cells. Following the addition ofthe manganese dioxide to the solution, conveniently in the leachingtank, an excess of base, for exapmple, zinc oxide in the 'form of 'zinccalcine, is added to the solution, thus neutralizing any residual acidand then, byy

further addition, causing precipitation -of the ironas ferrichydrate orsimilar insoluble iron compound: should, arsenic and/0r antimony bepresent they will be precipitated with the iron: At this point all ofthe zinc which is soluble is in solutionaszinc sulfate in the leachingtank; additionally, relative ly -small amounts of copper and'cadmium andpossibly other metals such as nickel and cobalt -are also in solution assulfates, these metals having been'leached out of the concentrate.

The contents of the leach tank may then bewpassed to a suitableclassifierwhich sepai rates the coars'e'r particles from the slime andthe former may then be 'carried through a ball mill or other suitabledevice to grind them toa slimewvhich is returned to the classifier asshown, these slimes constituting the final state of division of thesolids remaining from the leaching operation. Fromthe classifier theslimes may be directed to a Dorr or other suitablefthickener in which alarge proportion of the liquid is separated from the solids; the`thickened "slimes may then be passed to a continuous suction filterwhere the final separationof the solids from the liquid is effected andthe, filtrate then merged with the liquid coming* from the Dorrthickener. p p

This solution usually contains, as previous! ly noted, some'copper and;cadmium in addi- V tion` to the major metal zinc andthese, namely, thecopper andl cadmium, are/next prel cipitated iroml the solution in anysuitable way, desirably by the addition of zinc dust,

and together with yany excess of the z inc dust then' separated from thesolution by means of a 'iilte or thelike and should any other impuritiessuch as lnickelv 'and cobalt be present in the solution they areremoved'y by any of i the standard methods of purification utilized inVthelelectrolytic zinc industry. The solids Nfrom the ylter maythenbesubjectedto an treatment -suitable for recovering the camiumand/ory copper as by-products aswell .as the zinc .dusttl 1isvtreatment forming in o part of the present invention; in thispermissible recovery o the cadmium and/or lcopper and other metals'asby-products, however, resides one ofthe' advantages'of our method,leading, as it does, to a larger monetary yield from each ton of oretreated than is possible when processes are employed which result in theloss of these by-products, such, for example, as the usual retortprocess.

' The purified zinc sulfate solution delivered from the filter isnowreadyto be fedto the izo4 4 mesme.

strike andJoperating cells, and may for con- As heretofore stated, thebath in the strike veniencebe termed the primary-solution. cell ispreferably maintained at a consider- While for convenience ofdescription we tably lower acidity than the bathin the operhave hereinreferred to employmentvr of they ,'ating cells thus, for. example,good'results ,usual single leachprocess in the preparalareobtainediwlien tliebatli in the former is tion yof the primary zinc.sulfate solution, it i maintained at an acidity of 4% to 5%, i. e.,

vwill be understood that, if desired, the ,dou- 4()`.to"50 'grammesofsulphuric acid per thouble'leachWprocess may be employed iisitead sandcubie'lceiitinietes and the bath in the and under certain conditionswillbe prelatter atan aciditynofl 71/% to 10%, i. e.,

ferred, the primary object in either case, of p from 75 to l0() grammesof sulpliuric acid-per course, being to obtain at the lowest possiblethousandcubicl centimeters of solution when cost and in the mostconvenient way a substrike and operatingcuirents having density,

stantially puife neutral zinc sulfate solution respectively ofapproximately 150 and 60y am- Which is to` be fed to the strike andoperating'f peres per sq. ft. of cathode surface are to cells tomaintain the desired acidity therein be employed. .The particularacidity of the fandl replace the metallic zinc plated out/onY ytwobaths, however, isfc'apable of considerthe ferrous articles. The mannerof producable variation both aboveand below the pei'- -ing the neutralZine sulfate heretofore de- 4ceiitages indicated and will usually bedeter scribed does not in itself form any part of mined in vpractice byytheispcritic operating relatively lower density. In this -manner, cellshaving thus been brought to the desired 4electrolyti'c zinc andlithopone industries.

the present invention as the same is .well conditions present,yitb'eiiigof course appreknown and at present in common use in the ciated that,within limits, the higher the density of the" strike current withrelation In accordance with the preferred-practice to the acidity ofthe`bath the more rapid of the present invention the plating` on the willbe the [deposition of the strike coating iron oiisteel article iseffected in twobi' more and consequently the yshorter interval persteps`respectively performed in the strike mitted for theI evolution ofhydrogen. The

' coating cell with a relatively'low'l acid solu- ,f addition to thebaths of a small-amount of 'andthen in one,or more opeiatingcellswithusually found advantageous. a solution of higher acidity and a currentof The solution in the strike and operating tion and acurrent ofrelatively high density some/suitable addition agent such as glue is las will hereinafter more fully'appear, weare degreeof acidity theiron orsteel article to able by the substantially momentary passage which thecoating is to be applied, having of the high densityA current. throughthe been given a suitable-surface conditioning as 'strike cell to veryrapidly effect air initial by immersion in an alkali cleaning solutionto strikecoating of zinc upon thearticlevwitheffect grease removal,followed by rinsing out the evolution of an undue 'amount of hy- ,i andby pickling to remove scale, followed by drogen and with high currenteiticiencyand, Vrinsing and brushing again followed by alafter theformation of this ycoating to thicken kali cleaningy orelectro-alkalicleaning if the sameY to any required'degree by transdeemed desirable,is suspended in the soluporting the initially coated article't'o theoption inthe strike coating cell to act as a erating cell orcellsandpassing therethrough cathode` and a direct current passed thereto for alonger period, adem-rent of relatively through the bath from aninsoluble anode also lower 'current density, insoluble anodes, de-`suspendedfin the solution Vand desirably of linsoluble anodeto solutionto a proper operating value. zinc sulfate solution with which the cellsl-apparent that because of the relatively short sirablyof lead, beingemployed in both/the lead. As stated, the density of the strike strikeandoperating cells. y A current will be governed` more or less byVVhile'the method isin operation the) baths operating conditions and thelike, it being, in thestrike and operating cells areirespechowever,desirab-le to use a current of as high tively automatically maintainedin-properly density as possible within the bounds ofecoacidfconditionjas will hereinafter appear but nomic'goperation, forexample, 150 amperes must be initially brought to" that condition persql ft. ofk cathode surface, a current of by the addition of the properamount ,ofsulthisdensity being capable with a solution phuric acid or byassing a current from an having an acidity of 4% to 5% of depositingbuild up the acidity'of the /a very-satisfactory strike coating inapproxi- The mately one minute. It will therefore bey are initiallyfilled may be obtained in any time this current is flowing, the busbarsand 'desired way, for example, by forming a other yconnections andinstrumentalities by sufficient quantity of the primary solution inwhich isY carried may, without becoming the manner described bututilizing for the unduly .overloaded and heated, be of" matelll)leaching operation necessary therefor a di- -rially smaller size andcapacity than would lute sulphuricacid solution supplied from an berequiredvv were thecurrent directedthrough external source in suitablequantity instead themfor a materially longer interval, with of from theelectrolytic cellslas in the cyclic consequent decrease in cost ofinitial 'installaoperative performance of the method. l 'dom/plantmaintenance and fixed charges.

l As soon as the strike coating is formed, the strike current is shutoff and the article being plated transferred to the operating cellcontaining a bath of zinc sulfate of considerably higher acidity thanthat in the strike-coating cell and after the article has been immersedtherein a direct current of lower densit-y than employed in the strikecell is passed from an insoluble anode, desirably of lead, to thearticle acting as a cathode for an interval suiicient to form anultimate plating of the 'dcsired thickness on the article. Forsatisfactory and economical operation, a current having a density of 60amperes per sq. ft. of cathode surface may be employed with a bathhaving an acidity of substantially 10%, but

as in the case of the strike coating cell the acidity of the bath andthe density of the current are capable of considerable variation fromthese figures, while, of course, the length of time during which thecurrent is passed will be determined by the thickness of the 'plating itis desired to produce on the article.

As a guide, it may be stated that the application of60 amperes per sq.ft. of cathode surface with a solution containing 4% to 6% of sulphuricacid for a period of ten minutes will ordinarily deposit some 0.38ounces'of zinc per sq. ft. while under similar conditions theapplicationof the current for fifteen minutes will deposit a,considerably greater amount as from 0.50 to 0.7 ounces per sq. ft.depending upon the efiiciency of the current, but these amounts willnecessarily vary in accordance with different operative condi y tions.

soluble anodes to the iron or steel cathodes in the strike coating andoperating cells results in the deposition on their respective cathodes,that is, the articles to be coated which act as such, of metallic zincderived from the solution in the cells and the generation of a chem?solution being required to maintain a higher ferred practiceof ourinvention the efliuent acidity in the cell and a larger volume tomaintain a lower acidity for the same rate of acid generation. Inaccordance with the presolution or overflow from the strike coating cellis directed into the operating cell and asit isof lower acidity lthanthe .solution in the latter it to some extent tends to decrease the Asthe passage of the'current' from the inacidity thereof whilesimultaneously a suiiicient quantity of the primary solution is also fedto the operating cell so as to hold its acidity substantially constant,and the efliuent acid solution from this cell returned to the leachingtank or other point where the leaching operation is performed for thepurposr` of leaching additional zinc calcine. As the amount of acidcontained in this effluent solution is the substantial chemicalequivalent of the Weight of Zinc which has been deposited, a givenquantity of the solution when so returned to the leaching operationwill, for practical purposes, dissolve from thecalcines or other sourceof zinc supply a weight of zinc equivalent to that which was depositedwhile that quantity of the eluent solution was being discharged from theoperating cel?, so that the method is thus capable of being carried onuninterruptedly after being once putinto operation so long as sufficientzinc calcines or other zinkiferous materials are supplied to compensatefor the zinc deposited on the iron or steel articles which, from time totime as they receive their coatings of metallic zinc to the requiredthickness, are removed from the operating cells and after rinsing anddrying may be marketed without further treatment.

In the preferredA practice of our method as above described raw zincconcentrates are employed as the source of the zinc to be ultimatelyplated on the ferrous articles and, as heretofore pointed out, it isthus possible by the use of the method to effect a direct transferenceof the zinc in the ore to the articles and thereby avoid thedisadvantages necessarily vinherent in first recovering the zinc as ametal by the retort or electrolytic process and then utilizing thismetal for hot dipping or electroplating. However, under someconditionsit may be preferred to utilize metallic zinc derived in anysuitable way instead of zinc calcines in the leaching tank and to theresubject this metallic zinc, in a suitable state of division tofacilitate rapid dissolving, to the action of the eflluent acid solutiondrawn from the operating cell as hitherto described and in this mannerprovide the primary neuv tral Zinc sulfate solution for the strike andtinuous section filter shown on the flow sheet would ordinarily beunnecessary; equally so,

-if the metallic zinc supplied is devoid of cadmium, copper and/or otherimpurities, the addition of the zinc dust or other step designed'toeffect precipitation thereof may be omitted.

at the same current deisitare employed,

While under mostconditions but a single y.

a series of operating cells containing solui tions of different degreesof acidity and/or operated with currents of varying density.Additionally, while we prefer to gradually and continuously feed to thestrike coating cell sufficient zinc sulfate solution to maintain thesolution in the cell at a constant degree Aof acidity and to conduct theeliiuent acid solution displaced by the neutral zinc sulfate solutioneither to the operating cell or directly back to the leaching tank, itis also within the scope of our invention to refrain from feeding anyzinc sulfate solution to the strike coating cell and to permit theacidity of its solution to gradually increase as the strike current isintermittently carried'therethrough and to vary the densitybf thiscurrent in accordance with the increasing acidity of the cell instead ofoperating with a strike current of constant density as hithertodescribed.

Under certain circumstances it may be deemed advantageous to utilize buta single plating cell instead of separate strike and operating cells andto initially employ in this cell a relatively low acid solution as, forexample, one containing4% or 5% of sulphuric acid so that a strike orinitial coat- -ing can be formed on the ferrous article in that-cellwith high current eiiiciency by momentarily passing a current ofrelatively high density as, for example, 150 amperes per sq. ft. ofcathode surface, .the operation in this case being similar to thatemployed in forming the strike coating in accordance with the preferredpractice of the invention. Following the formation of the strikecoating, however, instead of transferring the article to another cellhaving a bath of higher acidity and then continuing the formation of thecoating therein Aby the use of a current of relatively lower density,the article may be allowed to remain in the bath and the passage of thecurrent continued, its amperfar as the character of the ultimate coatingis concerned, the results contemplated by our" invention, but We dov notconsider this method of operation as advantageous as the preferredpractice of the invention to which We have hitherto referred for thereasons, among others, it is ordinarily less economical and does notlend itself as readily to the desirable cyclic method of operation.

It will of course be understood that the instrumentalities .employed inthe practice of our improved method may be of any desired form suitablefor the performance of their various functions.

In the light of the foregoing disclosure of our invention the advantagesarising therefrom and to some of which brief reference has already beenmade can now more readily be appreciated and may in part be summarizedas (a) reduction in plant installation and operating costs and inreduction of fixed charges because of ability to utilize currentcarrying appliances of relatively small size and capacity; (b) increasedoperating eliiciency of the currents employed due largely to theshortening of the period of hydrogen evolution during the format-ion ofthe primary or initial coating' of zinc; (c) almost instantaneouscoating of the ferrous article with a strike coating of zinc so that buta negligible quantity of the iron or steel is dissolved in theelectrolyte before the article is protected from t-he action of thelatter; (d) the strike or incipient coating formed in the strike coatingcell upon the article greatly facilitates subsequent coating thereof inthe operating cell at economical current density and solution acidity;(e) the ability to employ a lrelatively low acid solution for theformation of the initial coating which in itself tends to diminish theevolution of hydrogen and decreases tendency of the .desired thick-nesson an iron or 'steel article by means of relatively high acid solutionsof zinc sulfate and currents of such character as may be handledeconomically within time intervals satisfactory for commercial work; (g)the character of the plating produced, exhibiting as it does substantialchemi cal purity, au extremely fine crystalline structure, greatdensity, good color and appearance, lack of porosity or sponginess. andpart-icularly markedly greater resista-nce to corrosion in the presenceof weak acids than the zinc coatings which it has hitherto been possibleto produce upon iron or steel articles of commerce by any process ormethod known to us.

Having thus described our invention, we

'claim and desire to protect by Letters Patent of the United States: A

1. In a method of clcctroplating zinc on ironor steel, the steps offorming a thin initial coating of zinc on the article to be plated bypassing thereto from an insoluble anode through a bath of a. solution ofzinc sulfate of relatively low acidity a. direct current of relativelyhigh current density and then afterv the formation of said initialcoating continuing the formation of the plating by the passage of a.current of lower density tothe article from an insoluble anode throughva bath of higher acidity than the first mentioned bath.

IVX

2. In a method of electroplating zinc on iron or steel, the steps offorming aI thin initial coating of zinc on the article to be plated bypassing thereto from an insoluble anode through a bath of a solution ofzinc sulfate containing at least 4% of sulphuric acid a direct currentof relatively high current density, transferring the initially coatedarticle to another bath containing a solution of zinc sulfate of greateracidity, and then continuing the plating of zinc on the article thereinby passing through the second bath from an insoluble anode to thearticle a direct current of lower density than the first mentionedcurrent.

3. In a method of electroplating zinc on iron or steel articles, thesteps of electrolyzing a bath of zinc sulfate containing a relativelysmall amount of sulphuric acid by passing therethrough from an insolubleanode to the article acting as a. cathode a current of relatively highdensity to thereby form on the article an initial relatively thincoating of zinc without excessive hydrogen evolution and then continuingthe formation of the coating by electrolyzing a bath of relativelygreater acidity by passing therethrough for a longer period a current ofrelatively lower density from an insoluble anode to the article actingas a cathode.

4. In a method of electroplating zinc on iron or steel articles, thesteps of electrolyzing a bath of zinc sulfate containing a relativelysmall amount of sulphuric acid by passing therethrough from an insolubleanode to the article acting as a cathode a direct current of relativelyhigh density to thereby form on the article an initial relatively thincoating of zinc without excessive hydrogen evolution, then continuingthe formation of the coating by electrolyzing a bath of relativelygreater acidity by passing therethrough for a longer period a directcurrent of relatively lower density from an insoluble anode to thearticle acting as a cathode, and maintaining the acidity of said bathssubstantially constant by feeding thereto regulated quantities of aneutral solution of zinc sulfate.

5. In a method of electroplating zinc on iron or steel articles, thesteps of forming a relatively thin initial coating of zinc on thearticle by electrolyzing for a short interval a solution of zinc sulfaterelatively low in acid by passing a current of relatively high densitythrough the solution from an insoluble anode to the article acting as acathode, transferring the article after the formation of said initialcoating to a second solution of maintaining the acidity of said secondsolution constant by feeding thereto a predetermined regulated quantityof a neutral solution of zijne sulfate.

6. In a method of electroplatin zinc on iron or steel articles, thesteps o forming an initial relatively thin coating of zinc on thearticle by electrolyzing a lsolutionof zinc sulfate containingsubstantially 5% of sulphuric acid by momentarily passing therethroughfrom an insoluble anode to the article acting as a cathode a directcurrent having a density of substantially 150 amperes per sq. ft. ofcathode surface, then transferring the article to a second solution ofzinc sulfate containing substantially 10% of sulphuricacid, continuingthe formation of the zinc 0n the article therein by electrolyzing saidsolution by passing therethrough from an insoluble anode to the articleacting as a cathode a direct current having a density ofV substantiallyamperes per sq. ft. of cathode surface and maintaining the acidity ofthe said solutions substantially constant by feeding thereto sufficientneutral solution of zinc. sulfate to compensate for the sulphurie acidgenerated therein by the passage of the currents.

7. In a. method of electroplating zinc on iron or steel articles, thesteps of forming an initial relatively thin coating of zinc on thearticle by electrolyzing a solution of'zinc sulfate containingsubstantially 5% of sul-l phuric acid by momentarily passingtherethrough from an insoluble anode to the article `acting as a cathodea direct current having a density of substantially 150 amperes per sq.ft. of cathode surface, then transferring the article to a secondsolution of zinc sulfate containing substantially 10% of sulphuric acid,and continuing the formation of the zinc on the article therein byelectrolyzing said solution by passing therethrough from an insolubleanode tothe article acting as a cathode a direct current having adensity of substantially 60 amperes per sq. ft. of cathode surface.

In witness whereof we have aiiixed our signatures.

SIDNEY H. DAVIS. CARL O. ANDERSON. WILLIAM N. SMITH. HERBERT R. HANLEY.

zinc sulfate of greater acidity and continuing the formation of thecoating on the article therein by passing through said solution from aninsoluble anode to the article acting asl a cathode a current of lessdensity for so long a time as may be required to form a coating ofthedesired thickness on the article and

